Phagocytosis of living Mycobacterium bovis (BCG and Ravenel strains) has been studied in suspension cultures of normal rabbit alveolar macrophages. Simultaneous fixation of the cell suspensions with glutaraldehyde and osmium tetroxide consistently demonstrated the presence of a clear zone around ingested mycobacteria and a double phagosomal membrane. Intracellular organisms remained morphologically intact up to 40 hours of incubation. The possible relationship of the double phagosomal membrane and lysosome-phagosome fusion mechanism will be further investigated. Special emphasis will be made to compare membranes of phagosomes induced by virulent and avirulent mycobacteria including M, smegmatis, M. phlei, H37Rv and H37Ra strains of M. tuberculosis, and the BCG and Ravenel strains of M. bovis. The experiments will include both in vitro and in vivo protocols to verify that the in vitro results are valid. Companion experiments will include the above protocols utilizing polymorphonuclear leukocytes and immunologically activated pulmonary macrophages. If indicated, additional experiments will employ circulating monocytes and human alveolar macrophages.